The field of this invention relates generally to measurement of the neutron flux within a nuclear reactor core.
Neutron sensitive detectors have for some time been the primary sensors used as incore neutron monitoring systems in conventional nuclear fission Light Water Reactors (LWR). The gas filled fission chamber has emerged as the commonly used sensor for Boiling Water Reactor (BWR) incore instrumentation, whereas the self-powered detector (SPD) is used in Pressurized Water Reactor (PWR) cores. U.S. Pat. No. 4,121,106 to Terhune and Neissel, entitled "Shielded Regenerative Neutron Detector", discloses an ion chamber type neutron detector; this patent is hereby incorporated by reference into this specification. U.S. Pat. No. 3,760,183 to Neissel, entitled "Neutron Detector System", discloses a combination of ion chambers and self-powered detectors; this patent is hereby incorporated by reference into this specification.
Alternatives to fission chambers exist in the art. One of these for example is the gamma sensitive ion chamber, now in use to calibrate the fixed incore power range sensors during power plant full power nuclear reactor operation. However, these sensors are expensive, difficult to manufacture, and somewhat delicate for general use. Another form of sensor, the previously mentioned SPD, is used in PWR's primarily for nuclear fuel management and steady state power distribution measurements. Unfortunately, SPD's do not have a prompt response, since the isotopes commonly used in the emitter electrode of the SPD typically have half-lives on the order of minutes. This precludes use of SPD's in BWR's, since the collective sensor signal is used for prompt safety functions, as well as fuel performance and power distribution monitoring. However, in those non-transient applications mentioned above, SPD's are simple in design and structure, reliable, inexpensive and long-lived.
Other approaches are much less widely used. Thermocouple sensors, in which the output signal is a function of the local gamma ray flux and therefore a measure of local power within the reactor core, have been developed and applied in European PWR's. An example is the gamma couple sensor (gamma thermometer). Unfortunately, these devices suffer from a low signal level output, and the attendant noise problems that limit their accuracy. Their response is not prompt, but can be fast enough by design that electronic and computer deconvolution methods can be applied to derive the prompt component of the gamma ray flux. These methods are very noise sensitive, and are usually too inaccurate for use in LWR safety functions. Nevertheless, the gamma couple sensor is simple, reliable, rugged, and cheap to manufacture. And unlike the ion chamber sensor, the gamma couple sensor has no gas filled volume with seals which can fail in service.
The fission couple is another thermocouple type sensor now in use. It is similar to the gamma couple, except that the source of heat for the fission couple is due to a fissionable isotope which is placed into intimate heat transferring contact with the thermocouple. As thermal neutrons induce fission in the fissile nuclei, energy is released in the form of heat, to thereby heat a locally placed thermocouple. This heating is a function of the local neutron flux. Additionally, the temperature of the fission couple is a function of the local gamma ray flux in a similar way as the gamma couple.
Advantages of these devices are that they are rugged, require no seal, and are relatively cheap and simple to manufacture. They also do not have to be powered by a voltage source because they are self-powered in the sense that a thermocouple is self-powered. Additionally, low impedance electronics can be used. Unfortunately, the component of the fission couple signal which is a function of gamma ray flux manifests itself as non-linearities in the output signal, even though the signal is typically very much larger than in a comparably sized gamma couple. Also, sensitivity of the fission couple to gamma rays limits the life of the fission couple, due to the 5:1 criteria of prompt-to-delayed signal ratio applicable to BWR safety system sensors. Fission couple lifetime is also shortened by the small size of the fissionable element, which must be small in order to obtain a reasonably fast response by the sensor. Lifetime is further shortened due to the burnup of the U-235 isotope contained in the absorber element.
Additional problems exist with present technology. Previous attempts to develop practical and useful fission couples have had limited success, principally because the sensors developed up to now have had slow response time, a short like, and produced a signal having poor linearity. Sensitivity has not been a problem, although it is known to be complementary to responsiveness.
Therefore, new or improved sensors are needed to measure the neutron flux within a nuclear reactor.